1. Field of the Invention
This invention relates to a motor control device and a motor control method for drive-controlling an AC motor.
2. Description of the Related Art
An electric vehicle having an AC motor as a driving force source is available in the prior art. In this electric vehicle, traveling drive torque is generated during travel by having the AC motor perform a power running operation, and regenerative braking torque is generated during braking by having the AC motor perform a regenerative operation.
Here, a driving system of the electric vehicle is constituted by a DC power supply, which is constituted by a secondary battery such as a lithium ion battery, an inverter circuit that is connected to the DC power supply and includes a capacitor and a plurality of semiconductor switches, and the AC motor, which is connected to the inverter circuit as a load.
The inverter circuit converts DC power from the DC power supply into predetermined AC power by switching the plurality of semiconductor switches ON and OFF at predetermined switching frequencies, and in so doing adjusts a torque and a rotation speed of the AC motor serving as the load. Further, depending on the operating condition thereof, the AC motor operates as a power generator so as to charge regenerative power generated by power generation to the DC power supply. Note that a highly efficient permanent magnet three-phase synchronous motor is often used as an AC motor applied to an electric vehicle.
In a driving system employing a three-phase synchronous motor, the inverter circuit is constructed by connecting three series circuits, each of which is configured such that an upper stage side switching element and a lower stage side switching element are connected in series, in parallel to the DC power supply. Further, respective midpoints of the three series circuits are connected to respective inputs of a U phase, a V phase, and a W phase of the three-phase synchronous motor.
Furthermore, by switching the switching elements provided in the respective phases of the inverter circuit ON and OFF in sequence, AC power is supplied to the respective phases of the three-phase synchronous motor at phases differing from each other by 120 degrees, and as a result, the three-phase synchronous motor is driven. Unless specified otherwise, it is assumed hereafter that the term “motor” denotes a three-phase synchronous motor. Note that operating principles of the inverter circuit are widely known, and will not therefore be described here.
Here, to protect the battery serving as the DC power supply from an overvoltage and an overcurrent, the driving system of the electric vehicle is provided with opening/closing means for disconnecting the battery from the inverter circuit as required. A condition for opening the opening/closing means is satisfied when a voltage of the battery equals or exceeds a predetermined value during a regenerative operation by the motor, when the battery voltage falls to or below a predetermined value due to battery wear, when a current flowing through the battery equals or exceeds a predetermined value, and so on. The opening/closing means may also be opened due to a fault in the vehicle, a collision, and so on.
In this driving system, the opening/closing means may be opened such that the inverter circuit is disconnected from the battery during the regenerative operation performed by the motor. Moreover, the inverter circuit may be disconnected from the battery even in a driving system not provided with the opening/closing means when a power line between the battery and the inverter circuit is cut.
In this case, the regenerative power flowing into the inverter circuit from the motor cannot be charged to the battery, and is charged to the capacitor of the inverter circuit instead. As a result, an overvoltage may be exerted on the capacitor, causing the capacitor to break.
Hence, when the inverter circuit is disconnected from the battery, six-switch opening processing may be executed to stop the inverter operation by switching all of the semiconductor switches of the inverter circuit OFF. When this six-switch opening processing is executed, however, power stored in a stator coil of the motor is charged to the capacitor via freewheel diodes (FWD) connected in reverse parallel to the switching elements, and as a result, an inter-terminal voltage of the capacitor may increase rapidly.
When, at this time, a capacitance and a voltage resistance of the capacitor are increased in order to cope with an increase in the inter-terminal voltage of the capacitor, the capacitor increases in size. Moreover, when the voltage resistance of the capacitor is increased, the voltage resistances of the respective constituent components of the inverter circuit must also be increased, making it difficult to achieve reductions in the size and cost of the inverter circuit. An inability to reduce the size of the inverter circuit is a particularly serious problem for an inverter circuit used in an electric vehicle, which must be installed in a limited space inside a vehicle.
Hence, to solve the problems described above, a method of adding a discharge circuit that consumes the regenerative power flowing into the inverter circuit from the motor by means of heat generation so that excessive regenerative power flowing into a capacitor is consumed by the discharge circuit has been proposed (see Japanese Patent Application Publication No. 2010-110099, for example).
A method of ensuring that power is not regenerated to the capacitor by executing three-phase short circuit processing, in which the respective phases of the motor are mutually short-circuited by switching all of the upper stage side switching elements or all of the lower stage side switching elements of the inverter circuit ON, when the inverter circuit is disconnected from the DC power supply instead of the six-switch opening processing has also been proposed (see Japanese Patent Application Publication No. H9-47055, for example).